A graphics processing unit (GPU) is incorporated in a central processing unit (CPU) that is a main processor of a personal computer (PC). Most notebook PC models use an internal GPU (hereinafter, referred to as an iGPU) incorporating a processor. A notebook PC is provided with an internal LCD, an external analog RGB output, an HDMI (registered trademark, omitted hereinafter) output and the like. These outputs are assumed by a display circuit composed mainly of an iGPU.
Some PC models are externally provided with a discrete GPU (hereinafter, referred to as a dGPU). However, a dGPU performs only so-called 3D operations represented by polygon processing, texture pasting, and light source or shading. A hybrid graphics function performed by an iGPU has been mainly used as a screen drawing function for a display output unit such as an internal LCD. Thus, all display outputs are basically governed by an iGPU
A conventional display unit has four or more display output ports, and a GPU output can be used directly for an internal LCD, an external DP, an external HDMI, and an external RGB. Specifically, in the cases of a DP, HDMI or RGB displays being connected as an external display unit, the GPU can immediately recognize each connection. Further, as for those displays showing their logos at the system power start-up time, it is possible to select a suitable display unit without specific concern of time.
An iGPU for a notebook PC is conventionally provided with an analog RGB output function in it. When this function is disabled, however, an external IC comes to be required to convert a digital display output to an analog RGB output. Therefore, for supporting an analog RGB output function with such an iGPU, an analog RGB output function is realized by providing a notebook PC with an IC that converts a DP signal to an analog RGB signal, if the iGPU has a digital display output function at its Display Port (hereinafter, referred to as a DP).
Such an IC detects connection of an analog RGB display unit upon its connection being made at an analog RGB output terminal, reads in an extended display identification data (EDID) such as resolution and refresh rate, and initializes itself in accordance with the connected display unit to obtain an appropriate DP output from an iGPU. The initialization requires about one second, and after completion of the initialization, the IC asserts a hot plug detect (hereinafter, referred to as a HPD) signal indicating that an external display unit becomes usable for an iGPU.
Thus, at the time where a conventional iGPU had an analog RGB output function, the iGPU can detect an external display unit immediately after connection. But, as for recent iGPU, an external display unit might be detected about one second after being connected.
Immediately after the PC is powered on, a display unit displays a user setting, and a logo window or a boot window that were previously selected according to a priority order specified by an OS. In a model supporting an analog RGB output, even when a user has previously connected an analog RGB with setting an analog RGB display as a preferential display unit, initialization of DP/analog RGB conversion IC would not be completed at the timing of BIOS (Basic Input and Output system) processing to show the selections of a boot display device after power-on since HPD would not be asserted. Thus, an iGPU does not detect an external analog RGB, and a logo is displayed in a display unit such as originally asserted as the next display priority (an internal LCD and the like).
There has been a demand for appropriate analog RGB display, but a means to realize such a demand has not been known.